The invention relates to apparatus for detecting iodine isotopes in the exhaust gas of a nuclear installation wherein a sampling flow is branched out of the main exhaust gas stream and conducted over an absorbent for measuring its iodine content.
In nuclear fuel reconstitution plants or other nuclear installations, exhaust gases must be monitored for their content of radioactive iodine isotopes, especially I-129. The activity monitoring, which may be performed on a continuous basis, is utilized to initiate measures for a reduction or elimination of the radioactivity in the exhaust gas or for an elimination of the causes for the problem, when abnormal increases in radiation occur. In this connection, it is known to take single samples manually, which samples are subsequently tested in a laboratory, as well as to provide for continuous sampling followed by laboratory analysis. The monitoring stations with continuous sampling are preferably arranged in the exhaust gas stack. The monitoring of exhaust gases from particular areas such as the dissolving plant and the first extraction cycle as well as a high activity waste storage area may be monitored separately.
These known methods are very time consuming and relatively bothersome. There is, especially, a long delay between the sampling and the sample evaluation which may result in a delay of about 8 hours for the introduction of countermeasures.
Evaluation testing needs to be performed, for example, in .alpha.-radiation shielded glove boxes.
A disadvantage of automatic sampling as it is known in the prior art resides in the fact that the degree of charge of the absorption materials with iodine isotopes cannot always be determined since an increase of the iodine content of the monitored gas appears sporadically, that is, for example, as a result of an accident. The absorption device always has to be ready for such accidents which means that the absorption material has to be exchanged after being charged to a certain degree, that is, upon reaching a certain radioactivity. It is, therefore, necessary to provide an optimal setting of detectors relative to the absorption devices which, for all practical purposes, represents a calibration or standard setting.
It is the object of the present invention to provide a monitoring arrangement of detector system, absorption device and exhaust gas flow which provides for optimal monitoring results.